[unreadable] G-protein coupled receptors (GPCRs) are a large superfamily of integral membrane proteins that transfer signals, via G-proteins, to downstream effectors. Detailed molecular information about how GPCRs activate their respective G-proteins is often lacking. Rhodopsin, a photoreceptor in the retina, defines the largest subfamily of GPCRs. As such, it acts as a paradigm for other receptors. Aberrations in rhodopsin's ability to couple to transducin (its G-protein) have been implicated in disease. The studies proposed here will provide direct structural information about the rhodopsin/transducin complex. Site-directed spin labeling (SDSL) experiments on both rhodopsin and transducin (Gt) will reveal regions of conformational change due to their mutual interaction. It is one goal of this proposal to identify areas of conformational change within transducin that are important for GDP release. Detailed analysis of these structural changes is expected to provide a mechanistic model for the rhodopsin catalyzed nucleotide exchange within Gt. Additionally, distance measurements will be made to map the binding site of transducin on rhodopsin. Collectively, these experiments will provide detailed structural information about the rhodopsin/transducin complex. [unreadable] [unreadable]